How much water do you need each day?
There is no single magic number that fits every body, job and climate, despite the popularity of simple rules about eight glasses. Needs rise with body size, heat, exercise, fever, and some health conditions, and fall on cool, quiet days. A practical estimate, and the stated method of this calculator, starts from body weight and then adjusts for the two biggest everyday factors: how long you exercise and whether the climate is hot.
Water also does not have to come only from a glass. Food provides roughly 20 percent of daily water for most people, and fruit, vegetables, soups, yoghurt and even rice and bread all contribute. Tea, coffee and other drinks count toward fluid intake as well. Use the figure on this page as a planning guide, then adjust with common-sense signals: thirst, a pale-yellow urine colour, and how you feel during and after activity. This page is for information only and is not medical advice.
The method used on this page
The base is 35 ml per kilogram of body weight, a widely used planning figure for adults in temperate conditions. Exercise adds 0.35 litres for every 30 minutes, reflecting typical sweat losses for moderate activity. A hot climate, or work that leaves clothing soaked with sweat, adds a flat 0.5 litres. Very long or intense exercise in heat can require more, plus electrolytes, which is beyond a simple daily estimate.
Reading the outputs
The result is shown in litres, US cups (1 cup = 236.6 ml) and 250 ml glasses, because different households picture amounts differently. The schedule below spreads your total across 16 waking hours to give a gentle hourly rhythm of roughly a small glass every hour or two. Drinking steadily usually feels better than catching up with a litre at once, and keeping water visible and within reach is the simplest habit that works.
Worked example
A 70 kg person who exercises for 30 minutes in a normal climate: base = 70 × 0.035 = 2.45 L; exercise adds 0.35 L; total = 2.80 litres. That is 2,800 ÷ 236.6 = about 11.8 US cups, or 2,800 ÷ 250 = 11.2 glasses of 250 ml across the day.
How to use this calculator
- Choose units and enter your weight.
- Enter your typical exercise minutes per day.
- Select hot climate if you live, work or train in significant heat.
- Read your daily total in litres, cups and glasses.
- Use the schedule below to spread drinks across your waking hours.
When needs are different
Some situations change water needs in ways a simple formula should not try to handle. Pregnancy and breastfeeding raise fluid needs. Kidney, heart and liver conditions sometimes come with fluid limits set by a clinician, and those instructions always override a general calculator. Older adults may feel thirst less strongly and benefit from a routine rather than waiting to feel thirsty. Children need proportionately different amounts and adult formulas should not be applied to them.
A reusable bottle makes the plan easier to follow: fill it in the morning, keep it where you work, and refill it a set number of times rather than trying to remember every glass you have had during a busy day.
More is not always better: drinking far beyond thirst over a short period can dilute blood sodium, a rare but serious problem seen mainly in endurance events. The safe pattern for most healthy adults is regular drinks, extra around exercise and heat, and attention to thirst and urine colour. If you are unsure about your own fluid needs for a medical reason, ask a qualified professional. Information only, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.